Field of the Invention:
The present invention relates to a method for the operation of a multilevel power converter for the reactive power compensation of a multiphase grid voltage, wherein a phase module including multiple submodules connected in series, each including an electric energy store, is associated with each phase, wherein each submodule may be connected to the phase of the grid voltage or may be disconnected from it with the aid of an electronic switch, and including a circuit breaker for disconnecting the power converter from the grid voltage.
Power converters of this type are used for the compensation of reactive power in an electrical grid. In this way, the voltage quality and stability in the power supply grid are improved, so that fluctuating energy sources such as offshore wind farms or photovoltaic systems may be connected to an electrical grid.
Power converters of this type are provided by the applicant under the name of SVC PLUS. These are modular multilevel power converters which are capable of generating a virtually sinusoidal voltage. The power converter is made up of multiple phase modules, one phase module being provided for each phase. Each phase module comprises a plurality of energy stores connected in series which act as a voltage source. The many voltage sources connected in series act as a voltage divider, via which the desired grid voltage having a sinusoidal profile is generated. Each of these voltage sources may include an energy store designed as a capacitor and a switch designed as an IGBT (insulated-gate bipolar transistor). Control electronics switch the power transistors in such a way that the capacitor may be bridged or activated as required, so that practically any desired voltage may be set.
In addition, the multilevel power converter includes a circuit breaker via which a disconnection of the power converter from the grid voltage may take place, for example, if a fault occurs in a device connected to the power converter. As soon as the power converter is disconnected from the three-phase grid voltage, the electric energy stores, preferably capacitors, in the submodules of the phase modules discharge unequally. This unequal voltage distribution also exists during reconnection, thus impeding the transition to controlled operation. In practice, several minutes must therefore elapse until the electrical energy stores in the submodules are almost completely discharged. In this operating state, after a disconnection via the circuit breaker, the power converter is not able to perform its function of stabilizing the grid voltage. In these cases, it is therefore necessary to revert to other devices, the use of which, however, has disadvantages such as the generation of severe grid distortions and a low dynamic response.